1. Field of the Invention
The present invention relates to a portable analog communication device having the capability of filtering voice and data signals which have been received or are to be transmitted.
2. Description of the Related Art
In a portable analog communication system, the frequency division multiplex technique is employed to utilize communication channels in a highly efficient manner. For example, in the case of the analog cellular telephone system of AMPS (Advanced Mobile Phone Service) serviced in North America, frequencies 869.04+0.03*n MHz are assigned to receiving channels while frequencies 824.04+0.03*n MHz (n=0 to 831) are assigned to transmitting channels. In such the system, the frequency bandwidth available for each voice communication channel is as narrow as 300-3400 Hz. Therefore, a filter is required to remove a high-frequency component from a signal. However, the known portable analog communication systems are designed for use in speech communication, and therefore the phase change caused by the filter is not taken into account in the design. In recent years, however, data communication using a modem has become popular, and a portable analog telephone device is now often used in data communication. However, the data rate via the portable analog telephone system is as low as 2400 bps. The low data rate mainly results from the following three factors. They are fading, hand-over, and group delay. The fading refers to a voltage (electric filed) variation in a received signal which occurs owing to interference between multiple radio waves propagating via multiple transmission paths. The fading is a problem inherent in the mobile communication. A sudden reduction in the electric field due to the fading causes a reduction in the signal-to-noise ratio, which in turn results in an increase in BER (bit error rate). In the case where the data is character data, the error will produce an incorrect character in the received data. The hand-over refers to the operation of automatically switching a telephone call when a portable telephone moves from a service area of a base station to an adjacent service area. The data lost during a switching time period is transmitted again.
The improvement of the problem associated with the group delay will be discussed below. First, a group delay caused by a filter will be discussed. In the case of a first-order LPF (low pass filter) shown in FIG. 10, the current I passing through the filter can be written as: EQU I=(Vin-Vout)/R=Vout/(1/j.omega.C)
where Vin denotes the input voltage (in V), Vout the output voltage (in V), R the resistance, C the capacitance, and .omega. the angular frequency (in rad/s). From the above equation, the transfer function T(j.omega.) can be obtained as follows: EQU T(j.omega.)=Vout/Vin=1/(1+j.omega.CR) (1)
If cutoff frequency .omega.o=1/CR is introduced into the above equation, then the characteristics associated with amplitude .vertline.T(j.omega.).vertline., phase .angle.T(j.omega.), and group delay .tau.(j.omega.) are given as follows: EQU .vertline.T(j.omega.).vertline.=1/.sqroot.(1+(.omega./.omega.o).sup.2)(2) EQU .angle.T(j.omega.)=-arctan((.omega./.omega.o)) (3) EQU .tau.(j.omega.)=-d[.angle.T(j.omega.)]/d.omega. (4)
The cutoff frequency o refers to a frequency at which a 3-dB reduction occurs in the amplitude .vertline.T(j.omega.).vertline.. Filters cause a change not only in the amplitude characteristic .vertline.T(j.omega.).vertline. but also in phase characteristic .angle.T(j.omega.). In the case of the above specific example, as can be seen from equation (3), a delay of 90 occurs for .omega.&gt;&gt;.omega.o. A great phase delay occurs near the cutoff frequency .omega.o. The group delay .tau.(j.omega.) is obtained by differentiating the phase .angle.T(j.omega.) with respect to the angular frequency .omega., and thus the group delay .tau.(j.omega.) indicates the degree of change in phase. Therefore, the group delay becomes large near the cutoff frequency .omega.o. The group delay becomes approximately 500 .mu.sec at .omega.o with respect to the delay for DC (0 Hz). A greater attenuation can be obtained by increasing the number of stages constituting a filter. However, the variation in phase and the group delay also increase with the number of stages. The transfer function is usually represented using a complex variable s. Thus, equation (1) becomes: EQU t(s)=1/(s+1) (5)
This is the general expression of the first-order LPF.
FIG. 11 is a block diagram of the receiver section of a known portable analog telephone device having the capability of data/facsimile communication. In this figure, 1a denotes an RF demodulator which demodulates a received signal in a baseband signal; 2 denotes an audio frequency band-pass filter which extracts a signal component in an audio frequency band 300-3400 Hz from a baseband signal and selectively outputs the resultant signal; 3a denotes a voice signal processing circuit called a de-emphasis/expander that performs a predefined receiving operation; and 4a is a selection switch for selecting a device the output signal of the voice signal processing circuit 3a is provided to. In speech communication, the output of the voice signal processing circuit 3a is connected to a loudspeaker circuit 5 via the switch 4a. In data/facsimile communication, the switch 4a provides the output signal of the voice signal processing circuit 3a to a modem 7 and further to a personal computer 9 via a transceiver 8 with an RC-232 and V.28 communication interface. The processing described above is controlled by a central processing unit 6. A portable analog telephone device includes an interface terminal for interfacing an external device such as a headphone or an optional device usually called a hands-free adapter. A user can also use the interface terminal when performing data/facsimile communication.
Now, the operation will be described with reference to the figures. A portable analog telephone device such as that shown in FIG. 11 has been originally designed for use in speech communication. Therefore, the audio frequency band-pass filter 2 is designed without taking into consideration the group delay distortion within the passband. Instead, much consideration about the amplitude attenuation characteristic outside the passband is taken in the design so as to minimize the influence on adjacent channels. As a result, group delay distortion of 3 to 5 ms usually occurs within the audio frequency band. Among the overall group delay which occurs in the audio frequency circuits, the majority of the group delay occurs in the audio frequency band-pass filter 2. On the other hand, in data/facsimile communication, the group delay distortion which occurs on a transmission channel has a great influence on the characteristics of a data signal. If the group delay distortion becomes large to a level which can no longer be neglected with respect to the data bit rate, interference between data bits occurs, which causes distortion in the signal. As a result, incorrect reception due to a data error occurs.
FIG. 12 is a block diagram of a transmitter section employed in a known portable analog telephone device having the capability of data/facsimile communication. As shown in FIG. 12, a voice signal, which is given in the form of an electric signal via a microphone 11, is directed via a switch 4b to an audio frequency band-pass filter 10, which in turn extracts a signal having frequency components in the range from 300 to 3400 Hz and outputs the resultant signal to a voice signal processing circuit 3b including a preemphasis circuit, a compressor and a limiter. The voice signal processing circuit 3b also includes a filter for removing harmonic components and thus extracting a low-frequency component from the input signal. This filter is designed to have a large cutoff frequency and to be of rather low order so that the group delay in the audio frequency range (300 Hz to 3.4 kHz) becomes low enough. The output signal of the filter is then modulated by an RF modulator 1b and is output as an RF signal. When data/facsimile communication is performed, the data output from a personal computer 9 is input to a modem 7 via a transceiver 8 with an RC-232/V.28 communication interface. The output signal of the modem is directed via the switch 4b to the audio frequency band-pass filter 10. After that, the signal is passed through the same path as the voice signal and is finally output as an RF signal. The switch 4b, the audio frequency band-pass filter 10, the voice signal processing circuit 3b, and the RF modulator 1b are all controlled by the central processing unit 6. In the transmitter section, as in the receiver section, the audio frequency band-pass filter 10 is designed with a great consideration on the amplitude attenuation characteristic outside the passband. As a result, group delay distortion of 3 to 5 ms usually occurs within the audio frequency band range from 300 to 3400 Hz.
In the known portable analog telephone device, as described above, when data/facsimile communication is performed, data is passed through the same signal path as the existing path designed to process a voice signal. As a result, the data signal encounters group delay distortion which can cause an error in the data.